The aim of this project is to test a metric of sentence formulation ability, called the Formulation Index (FI). The FI is measured independently of Mean Length of Utterance (MLU), but seems more closely related to the acquisition of grammatical morphology than MLU. This is because the FI is a more direct reflection of how the child employs grammatical competence in sentence production than MLU. The FI uses disruptions in the smooth flow of sentence production as basic data. Because the FI is independent of the morphosyntactic structure of any particular language, it can be used with other languages. The project has two specific goals: (a) to confirm that FI is indeed a better indicator of the acquisition of grammatical morphology than MLU; and (b) that FI can be used successfully in Spanish.